This invention relates to a preparation for epidermis containing kojic acid and/or its derivative and an ultraviolet light absorbent, which further contains at least one member selected from the group consisting of fatty acid esters and fatty acid glycerides for improving stability and attaining lasting effectiveness of the kojic acid and/or its derivative.
As typical forms of preparation for epidermis, there are illustrated O/W (oil-in-water) emulsions and W/O (water-in-oil) emulsions, which are different from each other in water-to-oil composition ratio and physical properties but are both homogeneous preparations wherein oil phase or aqueous phase are stably emulsified and dispersed with the aid of a surfactant.
Kojic acid and its derivatives the inventor has long studied are known as useful agents having various excellent properties, as disclosed in Japanese Unexamined Patent Publication No. 55-157509, Japanese Examined Patent Publication No. S56-18569, S58-22151, S58-22152, S58-34446, S60-7961, S60-9722 and S60-10005, Japanese Unexamined Patent Publication No. S60-137253, Japanese Examined Patent Publication No. S61-10447 and S61-60801, Japanese Unexamined Patent Publication No. S62-5909, Japanese Examined Patent Publication No. S62-3820 and S63-27322, Japanese Unexamined Patent Publication No. H1-132502 and Japanese Examined Patent Publication No. H5-30422.
However, kojic acid and its derivatives (hereinafter these being in some cases merely referred to as "kojic acids") are also known as agents which themselves have difficulty in acquiring stability. Particularly when the kojic acids are incorporated in the aforementioned O/W emulsion or W/O emulsion, it requires a highly sophisticated technique to design a proper formulation. Hence, it has been a pressing subject with respect to formation of a preparation containing the kojic acids to develop a technique which provides the kojic acids-containing prepatation with enough stability to stand severe distributive machinery without giving unpleasant feeling upon application thereof to skin.
In the case of compounding the kojic acids in various preparations for epidermis, they are under the condition of being likely to be exposed to ultraviolet light to varying degrees which can be an external cause of their coloration or decomposition. Thus, it has been conducted to compound an ultraviolet light absorbent in a proper amount for depressing damages by irradiation with ultraviolet light.
Examples thereof are illustrated in, for example, Japanese Unexamined Patent Publication No. S62-108804 and S64-83008 and Japanese Examined Patent Publication No. H4-46924.
Many of such ultraviolet light absorbents have a problem with solubility and separate out in the preparation, and fail to fully exhibit their ultraviolet light-absorbing ability, leading to a deteriorated stability of kojic acid.
In order to overcome this defect, solubilizing agents have properly been used. However, the use of oily solubilizing agent in a large amount causes a problem of giving an unpleasant feeling such as sticking feeling upon application to skin.
In addition, nonionic surfactants, which are properly used as surfactants upon forming a preparation containing kojic acids for external application in view of depressing coloration, giving a pleasant feeling upon application and being harmless to skin, have weaker emulsifying power in comparison with ionic surfactants and suffer decrease in emulsifying power in the presence of a highly polar ingredient or by the influence of pH level. Therefore, in the kojic acid-containing preparation which is usually adjusted to 4 to 5 in pH, incorporation of a highly polar ultraviolet light absorbent causes the problem of deteriorating emulsion stability with time.